This invention relates, in general, to controls for adding toner to developer material, and more particularly, to a method and apparatus for forming a toner image and measuring the optical density of the image which density corresponds to toner concentration in the developer to which toner is to be added.
Generally in xerography, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process the xerographic surface is electrostatically charged uniformly across its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where the light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original image to be reproduced. The latent electrostatic image is developed by contacting it with a finely divided electrostatically attractable powder (toner). The powder is held in image areas by the electrostatic charges on the layer. It is then transferred to a sheet of paper or other suitable surface and affixed thereto to form a permanent print. The toner forms one constituent of a suitable developer material which usually comprises carrier material and pigment particles. As will be appreciated, toner depleted from the developer has to be replaced in order to maintain a certain degree of developability. This can be done either manually or automatically.
In the case of automatic developability control, the dispensing of toner which is added to the developer has been controlled by measuring the optical density of toner periodically deposited on a pair of NESA glass plates which form a channel therebetween through which some developer flows. A potential is applied first on one of the plates and then on the other. The voltage serves to attract the oppositely charged toner from the developer material as it passes between the plates. An optical sensor is provided to monitor the optical density of the toner deposited which provides signals which are employed to control the addition of toner.
The gap between the plates has been found to be too narrow when toner and carrier particles are used which are much finer than heretofore utilized, resulting in blocking of the gap. One solution to the blocking problem is to increase the spacing between the parallel plates. To compensate for the reduced strength of the development field, the period (0.2H.sub.z) of the normally applied square wave (340 volts) is doubled. With such an arrangement there is the concern that with the longer time delay, a decrease in toner concentration will not be sensed by the ADC until the next cycle and this might give rise to overcorrection and instability in toner concentration. Accordingly, it would be desirable to be able to increase the gap size without decreasing the field strength.